EP0231994B1

EP0231994B1 - Clutch-to-clutch coast downshift control for a motor vehicle transmission

Info

Publication number: EP0231994B1
Application number: EP87300058A
Authority: EP
Inventors: Robert Charles Downs; Larry Theodore Nitz; Frederick Kurt Reichert; Joseph Lee Wanamaker
Original assignee: Motors Liquidation Co
Current assignee: Motors Liquidation Co
Priority date: 1986-01-27
Filing date: 1987-01-06
Publication date: 1990-01-17
Anticipated expiration: 2007-01-06
Also published as: EP0231994A2; JPS62184269A; US4671139A; DE3761416D1; JPH0541860B2; CA1273424A; EP0231994A3

Description

This invention relates to a method of controlling clutch-to-clutch downshifting in an electronic controller for a motor vehicle automatic transmission when the vehicle is in a coast mode of operation.
Motor vehicle transmissions generally include an input shaft coupled to the vehicle engine, an output shaft coupled to the drive wheels, and selectively engageable gear elements for providing two or more forward speed ratios between the input and output shafts. The speed ratios are determined by the relative sizes of the gear elements, and are typically defined in terms of the expression N/No, where Ni represents the input shaft speed and No represents the output shaft speed.
In automatic transmissions, the engine is connected to the input shaft by way of a fluid coupling such as a hydrodynamic torque converter, and the gear elements which provide the various speed ratios are selectively activated by fluid-operated torque-establishing devices such as clutches and brakes. A brake can be of the band or disc type; engineering personnel in the automotive art refer to the disc-type brakes in transmissions as clutches or clutching devices.
Shifting from one forward speed ratio to another generally involves releasing (disengaging) the clutching device associated with the current speed ratio and applying (engaging) the clutching device associated with the desired speed ratio. The clutching device to be released is referred to as the off-going clutching device, and the clutching device to be applied is referred to as the on-coming clutching device. Shifts performed in this manner are referred to as clutch-to-clutch shifts, in that no speed-responsive or one-way clutching devices are used. The clutching devices are activated in accordance with vehicle speed and engine load conditions so that the transmission is upshifted to successively numerically lower speed ratios (Ni/N_o) as the vehicle speed is increased, and is downshifted to successively numerically higher speed ratios as the vehicle speed is decreased.
The present invention is concerned with achieving effective control of clutch-to-clutch downshifting in conditions in which the engine load is minimal and the vehicle speed is decreasing. This condition is referred to herein as a coast condition (that is, coasting), and includes situations in which the operator uses the service brakes to augment the rate of speed decrease.
In coast downshifts, the objective is to time the shift so as to minimise driveline disruption while maintaining engagement of a speed ratio that will provide adequate performance in the event that the vehicle operator terminates the coast condition by increasing the engine throttle setting. Ideally, this means that the transmission should be successively downshifted as the vehicle speed is decreased, and that each such downshift should be performed so that the engine speed the shift is substantially equal to the no-load engine idle speed.
A long neutral interval between the release of an off-going clutching device and the apply of an on-coming clutching device is unacceptable, because a finite amount of time is required to prepare the on-coming clutching device, such that the engine will accelerate in an unrestrained fashion (that is, there will be engine flare) if the vehicle operator terminates the coast condition by increasing the engine throttle setting. The control problem is further compounded by variation in the engine idle speed and the vehicle deceleration rate.
The present invention is thus concerned with a transmission control system for effecting clutch-to-clutch downshifts in vehicle coast conditions, wherein the engine speed after each downshift is substantially equal to the no-load engine idle speed, and neutral idle intervals are minimised.
The invention is also concerned with a transmission control as set forth above wherein the downshift timing is compensated for variations in the engine idle speed.
The invention is further concerned with a transmission control as set forth above wherein the downshifts are scheduled in relation to the vehicle deceleration rate.
To these ends a control method in accordance with the present invention comprises the features specified in claim 1.
A preferred embodiment of a control method in accordance with the present invention is implemented with an electronic control system that regulates the fluid pressure supplied to each of the clutching devices. Downshifts in the course of a coast mode of operation are effected by releasing the off-going clutching device at a time determined in relation to the vehicle speed and deceleration rate, and applying the on-coming clutching device at a predetermined time thereafter such that apply occurs when the engine speed is substantially equal to the transmission input speed in the downshifted speed ratio.
The predetermined time defines a neutral interval during which neither the off-going nor the on-coming clutching device is engaged and the engine speed assumes its neutral idle value. In the course of the neutral interval, the on-coming clutching device is filled with hydraulic fluid in preparation for engagement. The duration of the predetermined time is thus scheduled in relation to both the time required for the engine speed to return to its neutral idle value and the time required for the filling of the on-coming clutching device. At the end of the predetermined time, the engine speed and the transmission input speed in the downshifted speed ratio are at the neutral idle value, and the on-coming clutching device is applied to complete the shift. This sequence is repeated for each successive downshift until the coast mode of operation is terminated or the transmission has downshifted to its lowest speed ratio.
In accordance with a further aspect of the present invention, variations in the engine idle speed are compensated for by obtaining a measure of the engine idle speed in the course of each period of coast operation. More particularly, the speed ratio across the torque converter is monitored following the onset of coast operation, and the point of zero torque transfer thereacross is identified. At such point, the engine and transmission input shafts are rotating at the same speed, namely the engine neutral idle speed. Such speed is captured, and is used in the timing of downshifts which occur in the course of that coast period.
A further aspect of the present invention relates to the scheduling of downshifts in the course of a coast mode of operation. Broadly, successive ratio downshifts (such as 4-3 or 3-2 downshifts) are eliminated if the scheduled apply of the on-coming clutching device falls within the predetermined time period preceding a further downshift. In such cases, the deceleration rate is relatively high, and the neutral interval is extended until the coast mode is terminated or a downshift to the lowest speed ratio can be scheduled. In the drawings:

Figures la and 1b schematically depict a computer-based electronic transmission control system for carrying out a control method in accordance with the present invention;
Figure 2 graphically depicts characteristic engine and transmission operation in the course of a period of coast operation;
Figures 3 and 4 graphically depict successive single-ratio downshifts performed in conformity with the present invention;
Figures 5 and 6 graphically depict characteristic engine and transmission operation in the course of coast operation, and identification of the engine neutral idle speed in conformity with the present invention;
Figure 7 graphically depicts coast operation at a relatively high deceleration rate at which certain normally scheduled downshifts are eliminated; and
Figures 8 to 11 are flow diagrams representative of computer program instructions to be executed by a computer-based control unit shown in Figure 1 in carrying out the control method in accordance with the present invention.

With reference now to the drawings, and more particularly to Figures 1 a and 1b, reference numeral 10 generally designates a motor vehicle drive train including an engine 12 and a parallel- shaft transmission 14 having a reverse speed ratio and four forward speed ratios. The engine 12 includes a throttle mechanism 16 mechanically connected to an operator-manipulated device such as an accelerator pedal (not shown) for regulating engine output torque, such torque being applied to the transmission 14 by way of the engine output shaft 18.
The transmission 14 transmits engine output torque to a pair of drive axles 20 and 22 by way of a hydrodynamic torque converter 24 and one or more of a plurality of fluid-operated clutching devices 26 to 34, such clutching devices being applied or released according to a predetermined schedule for establishing the desired transmission speed ratio.
With reference now more particularly to the transmission 14, an impeller 36 forming the input member of the torque converter 24 is connected to be rotatably driven by the output shaft 18 of the engine 12 by way of an input shell 38. A turbine 40 forming the output member of the torque converter 24 is rotatably driven by the impeller 36 by means of fluid transfer therebetween, and is connected to rotatably drive a shaft 42. A stator (reactor) member 44 re-directs the fluid which couples the impeller 36 to the turbine 40, the stator being connected by way of a one-way device 46 to the housing of the transmission 14.
The torque converter 24 also includes a clutching device 26 comprising a clutch plate 50 secured to the shaft 42. The clutch plate 50 has a friction surface 52 formed thereon adapted to be engaged with the inner surface of the input shell 38 to form a direct mechanical drive between the engine output shaft 18 and the transmission shaft 42. The clutch plate 50 divides the space between the input shell 38 and the turbine 40 into two fluid chambers, namely an apply chamber 54 and a release chamber 56. When the fluid pressure in the apply chamber 54 exceeds that in the release chamber 56, the friction surface 52 of the clutch plate 50 is moved into engagement with the input shell 38, as seen in Figure 1, thereby engaging the clutching device 26 to provide a mechanical drive connection in parallel with the torque converter 24. In such case, there is no slippage between the impeller 36 and the turbine 40. When the fluid pressure in the release chamber 56 exceeds that in the apply chamber 54, the friction surface 52 of the clutch plate 50 is removed out of engagement with the input shell 38, thereby uncoupling such mechanical drive connection and permitting slippage between the impeller 36 and the turbine 40. The circled numeral 5 represents a fluid connection to the apply chamber 54, and the circled numeral 6 represents a fluid connection to the release chamber 56.
A positive-displacement hydraulic pump 60 is mechanically driven by the engine output shaft 18 by way of the input shell 38 and the impeller 36, as indicated by a broken line 62. The hydraulic pump 60 receives hydraulic fluid at low pressure from a fluid reservoir 64, and supplies pressurised fluid to the transmission control elements via an output line 66.
A pressure regulator valve (PRV) 68 is connected to the pump output line 66, and serves to regulate the fluid pressure (hereinafter referred to as line pressure) in the line 66 by returning a controlled portion of the fluid therein to the reservoir 64 via the line 70. In addition, the pressure regulator valve 68 supplies fluid pressure for the torque converter 24 via a line 74.
The precise design of the pump and pressure regulator valve is not critical to the present invention. A representative pump is disclosed in US-A-4 342 545, and a representative pressure regulator valve is disclosed in US-A-4 283 970.
The transmission shaft 42 and a further transmission shaft 90 each have a plurality of gear elements rotatably supported thereon, namely gear elements 80 to 88 supported on the shaft 42 and gear elements 92 to 102 supported on the shaft 90. The gear element 88 is rigidly connected to the shaft 42, and the gear elements 98 and 102 are rigidly connected to the shaft 90. The gear element 92 is connected to the shaft 90 by way of a free-wheel (one-way) device 93. The gear elements 80, 84, 86 and 88 are maintained in meshing engagement with the gear elements 92, 96, 98 and 100 respectively, and the gear element 82 is coupled to the gear element 94 by way of a reverse idler gear 103. The shaft 90, in turn, is coupled to the drive axles 20 and 22 by way of gear elements 102 and 104 and a conventional differential gear set (DG) 106.
A dog clutch 108 is splined on the shaft 90 so as to be axially slidable thereon, and serves to rigidly connect the shaft 90 to either the gear element 96 (as shown) or the gear element 94. A forward speed relation between the gear element 84 and the shaft 90 is established when the dog clutch 108 connects the shaft 90 to the gear element 96, and a reverse speed relation between the gear element 82 and the shaft 90 is established when the dog clutch 108 connects the shaft 90 to the gear element 94.
The clutching devices 28 to 34 each comprise an input member rigidly connected to a transmission shaft 42 or 90, and an output member rigidly connected to one or more gear elements such that engagement of a clutching device couples the respective gear element and shaft together to establish a driving connection between the shafts 42 and 90. The clutching device 28 couples the shaft 42 to the gear element 80; the clutching device 30 couples the shaft 42 to the gear elements 82 and 84; the clutching device 32 couples the shaft 90 to the gear element 100; and the clutching device 34 couples the shaft 42 to the gear element 86. Each of the clutching devices 28 to 34 is biased towards a disengaged condition by means of a return spring (not shown). Engagement of the clutching device is effected by supplying fluid pressure to an apply chamber thereof. The resulting torque capacity of the clutching device is a function of the applied pressure less the return spring pressure, hereinafter referred to as the working pressure P_w. The circled numeral 1 represents a fluid passage for supplying pressurised fluid to the apply chamber of the clutching device 28; the circled numeral 2 and the letter R represent a fluid passage for supplying pressurised fluid to the apply chamber of the clutching device 30; the circled numeral 3 represents a fluid passage for supplying pressurised fluid to the apply chamber of the clutching device 32; and the circled numeral 4 represents a fluid passage for directing pressurised fluid to the apply chamber of the clutching device 34.
The relative sizes of the various gear elements 80 to 88 and 92 to 100 are such that engagement of first, second, third and fourth forward speed ratios is effected by engaging the clutching devices 28, 30, 32 and 34 respectively, it being understood that the dog clutch 108 must be in the position depicted in Figure 1 for a forward speed ratio to be obtained. A neutral speed ratio, corresponding to effective disconnection of the drive axles 20 and 22 from the engine output shaft 18, is effected by maintaining all the clutching devices 28 to 34 in a released condition. The speed ratios defined by the various pairs of gear elements are generally characterised by the ratio of the turbine speed N_t to output speed No. Representative N_t/N_o ratios for the transmission 14 are as follows:
As indicated above, shifting from a current forward speed ratio to a desired forward speed ratio requires that the clutching device associated with the current speed ratio (off-going device) be disengaged and that the clutching device associated with the desired speed ratio (on-coming device) be engaged. For example, a shift from the first forward speed ratio to the second forward speed ratio involves disengagement of the clutching device 28 and engagement of the clutching device 30. As is explained below, the timing of such disengagement and engagement is critical to the attainment of high-quality shifting, and this invention is directed to a control for supplying fluid pressure to the various clutching devices 28 to 34 to consistently achieve high-quality coast downshifts.
The fluid control elements of the transmission 14 include a manual valve 140, a directional servo 160 and a plurality of electrically operated fluid valves 180 to 190. The manual valve 140 operates in response to operator demand, and serves, in conjunction with the directional servo 160, to direct regulated line pressure to the appropriate fluid valves 182 to 188. The fluid valves 182 to 188, in turn, are individually controlled to direct fluid pressure to the clutching devices 28 to 34. The fluid valve 180 is controlled to direct fluid pressure from the pump output line 66 to the pressure regulator valve 68, and the fluid valve 190 is controlled to direct fluid pressure from the line 74 to the clutching device 26 of the torque converter 24. The directional servo 160 operates in response to the condition of the manual valve 140, and serves to properly position the dog clutch 108.
The manual valve 140 includes a shaft 142 for receiving axial mechanical input from the operator of the motor vehicle in relation to the speed range which the operator desires. The shaft 142 is also connected to an indicator mechanism 144 by way of a suitable mechanical linkage, as indicated generally by a broken line 146. Fluid pressure from the pump output line 66 is applied as an input to the manual valve 140 via the line 148, and the valve outputs include a forward (F) output line 150 for supplying fluid pressure for engaging forward-speed ratios and a reverse (R) output line 152 for supplying fluid pressure for engaging the reverse-speed ratio. Thus when the shaft 142 of the manual valve 140 is moved to the D4, D3 or D2 positions shown on the indicator mechanism 144, line pressure from the line 148 is directed to the forward (F) output line 150. When the shaft 142 is in the R position shown on the indicator mechanism 144, line pressure from the line 148 is directed to the reverse (R) output line 152. When the shaft 142 of the manual valve 140 is in the N (neutral) or P (park) positions, the input line 148 is isolated, and the forward and reverse output lines 150 and 152 are connected to an exhaust line 154 which is adapted to return any fluid therein to the fluid reservoir 64.
The directional servo 160 is a fluid-operated device, and includes an output shaft 162 connected to a shift fork 164 for axially shifting the dog clutch 108 on the shaft 90 to selectively enable either a forward or a reverse speed ratio. The output shaft 162 is connected to a piston 166 that is axially movable within the servo housing 168. The axial position of the piston 166 within the housing 168 is determined according to the fluid pressures supplied to the chambers 170 and 172. The forward output line 150 of the manual valve 140 is connected via a line 174 to the chamber 170, and the reverse output line 152 of the manual valve 140 is connected via a line 176 to the chamber 172. When the shaft 142 of the manual valve 140 is in a forward-range position, the fluid pressure in the chamber 170 urges the piston 166 rightwardly as viewed in Figure 1 to engage the dog clutch 108 with the gear element 96, for enabling engagement of a forward speed ratio. When the shaft 142 of the manual valve 140 is moved to the R position, the fluid pressure in the chamber 172 urges the piston 166 leftwardly as viewed in Figure 1 to engage the dog clutch 108 with the gear element 94, for enabling engagement of the reverse speed ratio. In each case, the actual engagement of the forward or the reverse speed ratio is not effected until engagement of the clutching device 30 occurs.
The directional servo 160 also operates as a fluid valve for enabling the reverse speed ratio. To this end, the directional servo 160 includes an output line 178 connected to an electrically operated fluid valve 186. When the operator selects a forward speed ratio and the piston 166 of the directional servo 160 is in the position depicted in Figure 1, the passage between the lines 176 and 178 is cut off; alternatively, when the operator selects the reverse gear ratio, the passage between the lines 176 and 178 is open.
The electrically operated fluid valves 180 to 190 each receive fluid pressure at an input passage thereof from the pump 60, and are individually controlled to direct fluid pressure to the pressure regulator valve 68 or respective clutching devices 26 to 34. The fluid valve 180 receives line pressure directly from the pump output line 66, and is controlled to direct a variable amount of such pressure to the pressure regulator valve 68, as indicated by the circled letter V. The fluid valves 182, 184 and 188 receive fluid pressure, from the forward output line 150 of the manual valve 140, and are controlled to direct variable amounts of such pressure to the clutching devices 34, 32 and 28, as indicated by the circled numerals 4, 3 and 1 respectively. The fluid valve 186 receives fluid pressure from the forward output line 150 and the directional servo output line 178, and is controlled to direct a variable amount of such pressure to the clutching device 30, as indicated by the circled numeral 2 and the circled letter R. The fluid valve 190 receives fluid pressure from the line 74 of the pressure regulator valve 68, and is controlled to direct a variable amount of such pressure to the release chamber 56 of the clutching device 26, as indicated by the circled numeral 6. The apply chamber 54 of the clutching device 26 is supplied with fluid pressure from the output line 74 via the orifice 192, as indicated by the circled numeral 5.
Each of the fluid valves 180 to 190 includes a spool element 210 to 220 respectively, axially movable within the respective valve body for directing fluid flow between input and output passages. When a respective one of the spool elements 210 to 220 is in the rightmost position as viewed in Figure 1, the input and output passages are interconnected. Each of the fluid valves 180 to 190 includes an exhaust passage, as indicated by the circled letters EX, such passage serving to drain fluid from the respective clutching device when the spool element is shifted to the leftmost position as viewed in Figure 1. In Figure 1, the spool elements 210 and 212 of the fluid valves 180 and 182 are shown in the rightmost position interconnecting the respective input and output lines, while the spool elements 214, 216, 218 and 220 of the fluid valves 184, 186, 188 and 190 are shown in the leftmost position interconnecting the respective output and exhaust lines. Each of the fluid valves 180 to 190 includes a solenoid 222 to 232 respectively for controlling the position of its spool element 210 to 220. Each such solenoid 222 to 232 comprises a plunger 234 to 244 respectively connected to the respective spool element 210 to 220, and a solenoid coil 246 to 256 surrounding the respective plunger. One terminal of each such solenoid coil 246 to 256 is connected to ground potential as shown, and the other terminal is connected to an output line 258 to 268 of a control unit 270 which governs the solenoid coil energisation. As set forth hereinafter, the control unit 270 pulse-width-modulates the solenoid coils 246 to 256 according to a predetermined control algorithm to regulate the fluid pressure supplied to the pressure regulator 68 and the clutching devices 26 to 34, the duty cycle of such modulation being determined in relation to the desired magnitude of the supplied pressures.
Although the fluid valves 180 to 190 have been illustrated as spool-type valves, other types of valve could be substituted therefor. By way of example, and without imitation, valves of the ball and seat type could be used. In general terms, the fluid valses 1ao to 190 may comprise any three-port pulse-width-modulated valving arrangement.
Input signals for the control unit 270 are provided on the input lines 272 to 284. A position sensor (S) 286 responsive to movement of the manual valve shaft 142 provides an input signal to the control unit 270 via the line 272. Speed transducers 288, 290 and 292 sense the rotational velocity of various rotary members within the transmission 14, and supply speed signals in accordance therewith to the control unit 270 via the lines 274, 276 and 278 respectively. The speed transducer 288 senses the speed of the transmission shaft 42 and therefore the turbine or transmission input speed N_t; the speed transducer 290 senses the speed of the drive axle 22 and therefore the transmission output speed No; and the speed transducer 292 senses the speed of the engine output shaft 18 and therefore the engine speed N_e. The position transducer 294 is responsive to the position of the engine throttle 16, and provides an electrical signal in accordance therewith to the control unit 270 via the line 280. A pressure transducer 296 senses the manifold absolute pressure (MAP) of the engine 12, and provides an electrical signal to the control unit 270 in accordance therewith via the line 282. A temperature sensor 298 senses the temperature of the oil in the transmission fluid reservoir 64 and provides an electrical signal in accordance therewith to the control unit 270 via the line 284.
The control unit 270 responds to the input signals on the input lines 272 to 284 according to a predetermined control algorithm as set forth herein, for controlling the energisation of the fluid valve solenoid coils 246 to 256 via the output lines 258 to 268. As such, the control unit 270 includes an input/output (I/O) device 300 for receiving the input signals and outputting the various pulse-width-modulation signals, and a microcomputer 302 which communicates with the I/O device 300 via an address-and-control bus 304 and a bidirectional data bus 306. Flow diagrams representing suitable program instructions for developing the pulse-width-modulation outputs in conformity with the present invention are depicted in Figures 8 to 11.
The characteristic operation of the engine 12 and the transmission 14 during coast operation is graphically illustrated in Figure 2, in which the turbine and engine speed traces for each of the four forward speed ratios of the transmission 14 are depicted as a function of vehicle speed N_v. The traces 400 and 402 represent the turbine and engine speeds, respectively, for the highest (fourth-speed) ratio; the traces 404 and 406 represent the turbine and engine speeds, respectively, for the third-speed ratio; the traces 408 and 410 represent the turbine and engine speeds, respectively, for the second-speed ratio; and the traces 412 and 414 represent the turbine and engine speeds, respectively, for the lowest (first-speed) ratio.
The neutral idle speed Ni is the speed at which the engine 12 operates when the transmission 14 is in Neutral. The drive idle speed N_d is the speed at which the engine 12 operates when the vehicle speed is zero and any one of the clutching devices associated with a forward speed ratio is engaged.
When the onset of a coast operation is sensed, the control unit 270 releases the torque converter clutching device 26, if engaged, to permit operation of the torque converter 24. Thereafter, the relative speeds of the engine 12 and the turbine 40 reflect the direction of the torque being transmitted through the torque converter 24. Regardless of the speed ratio in effect, the release of the clutching device 26 initially allows the engine speed to rise above the turbine speed. Shortly thereafter, however, the engine speed decreases below the turbine speed as the engine begins to supply negative (braking) torque to the vehicle through the torque converter 24. The engine 12 continues to supply braking torque through the torque converter 24 until the engine slows to its neutral idle speed Ni. At such time, the turbine and engine speeds N_t and N_e coincide, and no torque is transmitted through the torque converter 24. The terms N₄, N₃, N₂, and N₁ on the vehicle speed axis represent the speeds at which such coincidence occurs. Thereafter, the turbine 36 begins rotating faster than the impeller 40, and the sign of the torque transmitted through the torque converter 24 reverses. If no downshift is performed, the turbine speed N_t decreases to zero with the vehicle speed N_v, and the engine speed N_e decreases to its drive idle value N_d.
In conventional transmission controls, downshifting during coasting is generally postponed until the vehicle speed, is relatively low so as to minimise the driveline disturbances associated with shifting. The disadvantage of such control is that the transmission is usually in the wrong speed ratio if the operator terminates the coast operation before the vehicle is brought to a stop by increasing the engine throttle setting. This degrades the performance of the vehicle because a speed ratio shift has to be performed before the engine torque is transmitted to the drive axles 20 and 22.
In contrast to the conventional controls referred to above, the control in conformity with the present invention performs successive downshifts in the course of coast operation so that the transmission is in a more suitable speed ratio if and when the operator terminates the coast operation. Moreover, the downshifts are scheduled so that the engine speed before and after each shift is substantially the same, and the driveline disturbance occasioned thereby is minimised.
Figure 3 graphically depicts successive 4-3, 3-2 and 2-1 downshifts during coast operation in conformity with the present invention. The engine and turbine speeds for the various speed ratios are identified by the traces 400 to 434 as in Figure 2; the actual engine and turbine speeds in the course of the coast operation coincide with portions of the traces 400 to 414, and are depicted by the heavy traces 416 and 418. The neutral idle and drive idle speeds Ni and N_d as well as the terms N₄, Ns, N₂ and Ni are set forth as in Figure 2.
A time axis parallel to the vehicle speed axis denotes the times to to ta. The time to corresponds to a point relatively early in the coast operation, and the time t₈ corresponds to the point at which the vehicle speed Nv is reduced to zero. The time t_i corresponds to the vehicle speed N₄; the time t₃ corresponds to the vehicle speed N₃; the time ts corresponds to the vehicle speed N₂; and the time t₇ corresponds to the vehicle speed N₁.
Between the times to and t₂, the fourth forward speed ratio of the transmission 14 is engaged, and the turbine speed N₁ and the engine speed Ne follow the traces 400 and 402 respectively. When Nt and Ne coincide at the time ti, the engine is operating at its neutral idle speed Ni. As is set forth below, the turbine (or engine) speed is measured at such time in conformity with the present invention to provide an indication of the current value of Ni.
Following the time t₁, the control unit 270 periodically predicts future turbine speed values PTS₃, PTS₂ and PTS₁ for the third, second and first-speed ratios of the transmission 14. The predicted turbine speed PTS_n for a given speed ratio n calculated at time t is an estimate of the turbine speed that would occur at time (t + Tp) if the speed ratio n were engaged, where Tp is a predetermined time interval. As is explained below, the predetermined time Tp is chosen in relation to the dynamics of the engine 12 and the required fill times of the various clutching devices.
The time t₃ represents the time at which the predicted turbine speed PTSs for the third-speed ratio coincides with the engine neutral idle speed Ni. Similarly, the time ts represents the time at which the predicted turbine speed PTS₂ coincides with Ni and the time t₇ represents the time at which the predicted turbine speed PTS₁ coincides with Ni. Thus, the intervals (t₃- t₂), (ts - t4) and (t₇ - te) each have a duration equal to the predetermined time Tp. In the illustrated embodiment, the predetermined time Tp has a value of 0.25 seconds.
The predicted turbine speeds are computed as a function of the vehicle deceleration a, the current vehicle speed N_v, the reference time interval Tp, and the Nt/No ratio defined by the respective speed ratios. Algebraically, the predicted turbine speed PTSn for a given speed ratio n is computed according to the following expression:
For the transmission depicted in Figure 1, the predicted turbine speeds PTS₁, PTS₂, and PTS₃ for the first, second and third speed ratios are thus given as follows:

The engine neutral idle speed Ni identified at time t₁ is continuously compared with the predicted turbine speed for the downshifted speed ratios. When Ni coincides with PTS_n, it means that a downshift to the speed ratio n should occur in Tp seconds. When such coincidence is detected, the clutching device associated with the currently engaged speed ratio is released, thereby permitting the engine speed to increase (float up) to its neutral idle value Ni. At the expiration of the time interval Tp, the engine speed should be substantially at the neutral idle value, and the clutching device associated with the speed ratio n is applied to effect the downshift.
Accordingly, the 4-3 downshift is effected by releasing the clutching device 34 at time t₂, and applying the clutching device 32 at time t₃. In the neutral interval between release and apply, the clutching device 32 is prepared for engagement, and the engine and turbine speeds rise to the neutral idle speed Ni. Correspondingly, the 3-2 downshift is effected by releasing the clutching device 32 at time t₄, and applying the clutching device 30 at time ts; similarly, the 2-1 downshift is effected by releasing the clutching device 30 at time t₆, and applying the clutching device 28 at time t₇. In the neutral interval between each release and apply, the clutching device associated with the downshifted speed ratio (on-coming device) is filled in preparation for engagement, as engine speed and turbine speeds N_e and Nt rise substantially to the neutral idle speed Ni. After the clutching device 28 has been applied to engage the first-speed ratio, the turbine speed Nt decreases to zero along the trace 412, and the engine speed N_e decreases to its drive idle value N_d along the trace 414.
The torque converter speed ratio Nt/Ne and the pressure commands for the clutching devices 28 to 32 during the 3-2 and 2-1 downshifts are also shown in Graphs A to E of Figure 4, in which the time designations t₄ to ts are as set forth in Figure 3. Graph A depicts the speed ratio N_t/N_e across the torque converter 24; Graph B depicts the fluid pressure command P(3) for the third-speed ratio clutching device 32; Graph C depicts the fluid pressure command P(2) for the second-speed ratio clutching device 30; Graph D depicts the fluid pressure command P(1) for the first-speed ratio clutching device 28; and Graph E depicts the engine speed N_e. As is seen in Graphs C and D, the filling of the clutching device 30 occurs in the interval t_f2- ts, and the filling of the clutching device 28 occurs in the interval t_f1- tτ.
In view of the above, it will be evident that the predetermined time Tp must be chosen in relation to two constraints. Firstly, the time Tp must be sufficiently long to permit the engine speed N_e to return to its neutral idle value from a somewhat lower value following the release of the respective off-going clutching device. Secondly, the time Tp must be sufficiently long to permit the on-coming clutching device to be filled. In any event, the time Tp should be as short as possible in order to minimise the neutral intervals. In a specific application of a control in accordance with the present invention in a production vehicle, the first constraint was found to be controlling. In this specific application, the time Tp was set at 0.25 sec. for all coast downshifts.
In each of the downshifts described above, the engine and turbine speeds before and after the apply of the respective on-coming clutching device are substantially the same. As a result, the clutching device does not have to overcome the inertia of the engine, and the driveline disturbance associated with the shift is minimised. Moreover, the performance of the vehicle at the termination of the coast operation is enhanced, because the successive downshifting in the course of the coast operation places the transmission in a more suitable speed ratio for effecting acceleration of the vehicle than would be the case with conventional controls.
As has been set forth above, a further aspect of the present invention relates to the identification of the engine neutral idle speed Ni in the course of each coast operation. If the neutral idle speed Ni remained constant throughout the vehicle operation, there would be no need to measure it, and all coast downshifting could be timed in relation to the vehicle speeds N₃, N₂ and N₁ set forth in Figures 2 and 3. However, the neutral idle speed of a motor vehicle engine varies significantly with temperature and accessory loading during a typical period of operation. Figure 5 graphically illustrates how variation in the neutral idle speed changes the optimum timing of a coast downshift.
More particularly, Figure 5 depicts turbine and engine speed traces for two different neutral idle speeds N; and Ni', assuming engagement of the fourth forward speed ratio. Corresponding drive idle speeds N_d and N_d' are also shown. The turbine and engine speeds N_t and N_e corresponding to the neutral idle speed Ni are depicted by the solid traces 400 and 402 as in Figures 2 and 3; the engine speed N_e' corresponding to the neutral idle speed Ni' is depicted by the broken trace 420. The turbine speed N_t is directly related to the vehicle speed and does not vary with the neutral idle speed. The turbine speed N_t for the third-speed ratio is depicted by the trace 404 as in Figures 2 and 3. On the vehicle speed axis, the term N₄ corresponds to the point at which the turbine and engine speed traces 400 and 402 coincide at neutral idle speed Ni as in Figures 2 and 3; the term N₄' corresponds to the point at which the turbine and engine speed traces 400 and 420 coincide at the neutral idle speed Ni'. If the engine neutral idle speed is N₁, the on-coming clutching device 32 for the third-speed ratio should be applied at vehicle speed N₃ in order to perform a minimum-disruption 4-3 downshift in conformity with the present invention. If the engine neutral idle speed is Ni', the clutching device 32 should be applied at a significantly higher vehicle speed N₃'. Thus, vehicle speed cannot be a basis for the timing of coast downshifting if the downshifts are carried out in conformity with this invention for achieving minimum driveline disruption.
In conformity with the present invention; the engine neutral idle speed is determined in the course of each coast operation by monitoring the speed ratio N_t/Ne across the torque converter 24 in the early portion of the coast, and identifying the point at which the ratio is unity -- that is, time t₁ in Figure 3. The mechanism for identifying the engine neutral idle speed in conformity with this invention is illustrated graphically in Figure 6, where the speed ratio Nt/Ne across the torque converter 24 is depicted as a function of time for a period of coast operation. For the purpose of illustration, it is assumed that the torque converter clutching device 26 is engaged prior to the coast operation. In such case, the engine torque is transmitted by way of the clutching device 26 rather than by way of the torque converter 24, and the speed ratio Nt/Ne across the torque converter 24 is 1:1, namely unity.
At time to, coast operation (closed throttle deceleration) is detected, and the control unit 270 releases the clutching device 26 to permit operation of the torque converter 24. Thereafter, the relative speeds of the engine 12 and turbine 40 reflect the direction of the torque being transmitted through the torque converter 24. The release of the clutching device 26 allows the engine speed to flare, and the ratio N_t/N_e decreases below unity, indicating that the engine is supplying some driving torque to the vehicle by way of the torque converter 24. Shortly thereafter, at time ti, the engine speed decreases, and the speed ratio N_t/N_e increases above unity as the engine begins to supply negative (braking) torque to the vehicle by way of the torque converter 24. At time t₂, the vehicle speed begins decreasing faster than the engine speed, and the ratio N_i/N_e begins decreasing towards zero. As the ratio N_t/Ne decreases, the engine supplies less and less braking torque, until at time ts the ratio reaches unity. At such time, the engine is at its neutral idle speed, and the impeller 36 of the torque converter 24 begins rotating faster than the turbine 40. As a result, the torque transmitted through the torque converter 24 reverses as the engine 12 once again begins supplying positive torque to the vehicle by way of the torque converter 24. If no downshift were performed, the turbine speed N_t, and therefore the ratio N_t/N_e, would thereafter decrease to zero as the vehicle slowed to a stop, as indicated by the broken trace 422. If a coast downshift were performed, the release of the clutching device 34 5 would permit the ratio Nt/Ne to float back to unity in the ensuing neutral interval, as shown by the solid trace and by the N_t/N_e trace of Graph 4A.
Identification'of the engine neutral idle speed Ni in conformity with the present invention in- 10 volves defining a zero-torque window about the torque converter speed ratio of unity, and capturing the engine or turbine speed as the ratio Nt/Ne passes through the window at time ts. Such window is defined, as seen in Figure 6, by the ratio 15 values HI and LO disposed about the ratio of 1.0. The term COASTHR is a calibrated value significantly greater than the upper window limit HI. When the measured ratio Nt/Ne becomes greater than the value of COASTHR in the course of a 20 coast operation, the first passage of the ratio through the zero torque window (time t_i) has already occurred, and the mechanism for capturing the engine neutral idle speed is enabled. The term DRIVETHR is a calibrated value significantly 25 lower than the lower window limit LO, and is used in connection with a check of the reasonableness of the stored neutral idle speed Ni. A specific application of the neutral idle speed capturing technique is described below in relation to the flow 30 diagrams of Figures 11 and 12.
In practice, the control unit 270 stores a running estimate of the neutral idle speed Ni. At the initiation of vehicle operation, an estimate of Ni is stored that is based on the expected operation of 35 the engine speed control system and typical accessory loading. In subsequent coast operation, the stored value is adjusted into agreement with the actual neutral idle speed, as is explained below in relation to the flow diagram of Figure 9b. 40
In instances in which the deceleration rate is relatively high, there may not be enough time to complete all the successive downshifts. In such case, certain of the downshifts may be skipped. The heavy trace 440 in Figure 7 represents the 45 engine speed in the course of coast operation under relatively high vehicle deceleration. The engine and turbine speed traces 400 to 414 from Figures 2 and 3 are also shown. As with the coast operation depicted in Figure 3, the fourth-speed 50 ratio is engaged initially at time to, and the engine speed trace follows the path of the engine speed trace 402. Also as in Figure 3, the engine neutral idle speed Ni is captured at time t_i as the engine and turbine speeds coincide. At time t₂, the pre- 55 dieted turbine speed PTS₃ for the third-speed ratio is substantially equal to the captured neutral idle speed, and the control unit 270 releases the clutching device 34. Normally, the clutching'device 32 for the third-speed ratio would be applied 60 at a time Tp seconds later, at time t₃. Due to the relatively high vehicle deceleration, however, the predicted turbine speed PTS₂ for the second-speed ratio becomes substantially equal to the neutral idle speed Ni prior to time t₃. Since the 65 release of the third-speed ratio clutching device 32 is indicated prior to its scheduled engagement, a 4-3 shift is not appropriate, and apply of the clutching device 32 is skipped. As such, the en-5 gine speed remains at the neutral idle value Ni.
If a 4-3 shift is not appropriate, the control unit 270 determines whether a 4-2 shift is appropriate. The apply of the second-ratio clutching device 30 would normally occur at time t₄, and thus at a 10 time Tp seconds after the scheduled release of the third-speed ratio clutching device 32. In the illustrated example, however, the predicted turbine speed PTS₂ for the second-speed ratio becomes substantially equal to the neutral idle 15 speed Ni prior to time t₄. Since release of the second-speed ratio clutching device 30 is indicated prior to its scheduled engagement, a 4-2 shift is not appropriate, and apply of the clutching device 30 is skipped. As such, the engine speed 20 remains at the neutral idle value Ni.
If a 4-2 shift is not appropriate, the control unit 270 effects a 4-1 shift at the time ts, and thus at a time Tp seconds after the point at which the predicted turbine speed PTS₁ for the first-speed ratio 25 became substantially equal to the neutral idle speed Ni. Such shift is effected by applying the first-speed ratio clutching device 28. Thereafter, the engine speed follows the engine speed trace 414 until the drive idle speed N_d is reached at 30 time te.
Figures 8 to 11 depict flow diagrams representative of program instructions to be executed by the control unit 270 for carrying out the control functions of this invention. The flow diagram of 35 Figure 8 represents a main or executive program which calls various sub-routines for executing particular control functions as necessary. The flow diagrams of Figures 9 to 11 represent the functions performed by those sub-routines which 40 are more pertinent to the control of the present invention.
With reference now more particularly to Figure 8, the reference numeral 470 designates a set of program instructions executed at the initia-45 tion of each period of vehicle operation for initialising the various registers, timers etc. used in carrying out the control functions of this invention. Following such initialisation, the instruction blocks 472 to 480 are repeatedly executed in sequence, 50 as designated by the flow diagram lines connecting such instruction blocks and the return line 482. The instruction block 472 reads and conditions the various input signals applied to the I/O device 300 via the lines 272 to 280, and updates 55 (increments) the various control unit timers. The instruction block 474 calculates various terms used in the control algorithms, including the predicted turbine speeds PTSn the vehicle acceleration a. the speed ratio N_t/N_e, and an engine 6,0 torque-related variable Tv. The algebraic expressions used to compute the predicted turbine speeds PTS_n are given above in relation to Figure 3. The instruction block 476 determines the desired speed ratio, R_des, this being a function 65 generally referred to as shift pattern generation. In non-coast operation, R_des is determined in a conventional manner in accordance with throttle position, vehicle speed and manual valve position; in coast operation, on the other hand, R_des is determined in conformity with the present invention, to achieve minimum driveline disruption downshifting.
The instruction block 478 determines the clutching device pressure commands for effecting a ratio shift, if required. The pressure commands for the pressure regulator valve PRV and non-shifting clutching devices are also determined. The instruction block 480 converts the clutching device and PRV pressure commands to a PWM duty cycle based on empirically determined operating characteristics of the various actuators, and energises the appropriate actuator coils accordingly.
The flow diagrams depicted in Figures 9 to 11 represent an expansion of certain of the main flow diagram instruction blocks. Shift pattern generation - instruction block 476 in Figure 8 -- is expanded-on in the flow diagrams of Figures 9a to 9c. Pressure command determination -- instruction block 478 in Figure 8 -- is expanded-on in the flow diagrams of Figures 10 and 11.
The shift pattern generation flow diagram of Figures 9a and 9b includes a coast downshift (CDS) enabling routine 490, a coast abort routine 492, an active coast testing routine 494, a neutral idle speed capture routine 496, and a CDS timing routine 498.
The coast downshift enabling routine 490 comprises the decision blocks 500 to 506 for defining the enabling conditions for a coast downshift. The decision block 500 determines whether the torque converter clutching device 26 is released; the decision block 502 determines whether the engine throttle position is less than a reference value, REF_tp, corresponding to a nearly closed position; the decision block 504 determines whether the vehicle acceleration is less than a relatively low reference, REF_a; and the decision block 506 determines whether the vehicle speed Nv is less than a relatively high reference, MAX. If all the decision blocks 500 to 506 are answered in the affirmative, the enabling conditions are met, and the active coasting routine 494 is executed to determine whether a coast condition should be established. If any of the decision blocks 500 to 506 are answered in the negative, the coast abort routine 492 is executed, to cancel the coast condition.
The coast abort routine 492 includes the blocks 508 to 514, and is executed when either the CDS enabling routine 490 or the active coast testing routine 494 indicates that coast downshift control is not appropriate. The instruction block 508 is first executed to reset the "COAST CONDITION" and "ACTIVE CDS" flags. As is set forth below, the status of the "COAST CONDITION" flag is determined by the active coast testing routine 494, and the status of the "ACTIVE CDS" flag is determined by the CDS timing routine 498. The instruction block 510 is then executed to determine the proposed speed ratio, Rprop, based on the vehicle speed Nv, the throttle position TP, and the position, MAN, of the manual valve 140. Then the decision block 512 is executed to determine whether the "SHIFT IN PROGRESS" flag is set. As is described below in relation to Figures 10 and 11, the "SHIFT IN PROGRESS" flag is set and reset by the shift control routines to indicate the status of a shift. If the "SHIFT IN PROGRESS" flag is not set, the instruction block 514 is then executed to set the desired speed ratio term R_des equal to the proposed ratio term Rp_rop; otherwise, execution of the instruction block 514 is skipped, so completing the routine.
The active coast testing routine 494 is executed after it has been determined that coast-enabling conditions are present. On entry into the routine, the decision block 516 is executed to determine whether the "COAST CONDITION" flag is set. If so, the active coast tests have already been met, and execution of the remainder of the routine is skipped, as indicated by the flow diagram line 518. If not, the decision blocks 520 to 526 are executed, to perform the active coast tests. The decision block 520 determines whether the desired speed ratio R_des is first; the decision block 522 determines whether the ratio Nt/N_e across the torque converter 24 is greater than unity; the decision block 524 determines whether the vehicle brake is applied; and the decision block 526 determines whether the vehicle speed Nv is less than a moderate reference speed REFmod.
If the desired speed ratio R_des is other than the first speed ratio and the ratio N_t/N_e is greater than unity, the instruction block 528 is executed to set the "COAST CONDITION" flag and to set the target coast downshift ratio Reds equal to one ratio lower than the present ratio Pact. If the desired speed ratio R_des is first, a coast condition cannot be established, and the coast abort routine 492 is executed, as indicated by the flow diagram line 530.
Similarly, if the ratio N_t/N_e is less than unity and the vehicle brakes are not applied, or the vehicle speed is greater than REF_mod, a coast condition cannot be established, and the coast abort routine is executed, as indicated by the flow diagram lines 532 or 534. In such case it is not necessary to execute the instruction block 508, since neither the "COAST CONDITION" flag nor the "ACTIVE CDS" flag would have been set. If the vehicle brakes are applied and the vehicle speed is less than REFmod, the instruction block 528 is executed to set the coast downshift ratio Reds to (R_act - 1), and to set the "COAST CONDITION" flag even though the ratio Nt/Ne indicates that positive torque is being transmitted by way of the torque converter 24.
Once the "COAST CONDITION" flag has been set, indicating that CDS conditions are present and that the active coast tests have been met, the neutral idle speed capture routine 496 is executed. Initially, the decision block 536 is executed, to determine whether the desired ratio R_des is first. If R_des is first, and the first-speed ratio is engaged, as determined at the decision block 537 by comparison of R_des with Pact the instruction block 538 is executed to clear the "ACTIVE CDS" flag.
If the desired speed ratio R_des is other than the first-speed ratio, the decision block 540 is executed to determine whether the "ACTIVE CDS" flag is set. If so, the neutral idle speed Ni has already been captured, and the remainder of the routine is skipped, as indicated by the flow diagram line 542. If not, the decision block 544 is executed, to compare the ratio N_t/N_e with the reference term COASTHR, defined above in relation to Figure 6.
If the ratio N_t/N_e is at least as great as COASTHR, the instruction block 546 is executed to set the "COASTBIT" flag, indicating the impending passage of the ratio N_t/N_e through a value of unity. In such case, the decision blocks 548 and 550 are executed, to compare the ratio N_r/N_e with the reference terms HI and LO (also defined in reference to Figure 6), for determining whether the ratio is within the window defined thereby. If the ratio N_c/N_e is within the window and the "COASTBIT" flag is set, as determined at decision block 552, the instruction block 554 is executed, to clear the "COASTBIT" flag and to average the current turbine speed N_c into the stored neutral idle speed Ni.
If the ratio N_c/N_e is not within the window or if the "COASTBIT" flag is not set, the execution of instruction block 554 is skipped, as indicated by the flow diagram line 556. Thus the neutral idle speed Ni is only captured after the ratio N_t/N_e has exceeded the term COASTHR. As is indicated above, the neutral idle speed is estimated and stored in the control unit 270 when the engine is started. In subsequent operation, the stored neutral idle speed Ni is adjusted by the averaging technique of the instruction block 554.
The neutral idle speed capture routine 496 also includes provision for determining the reasonableness of the stored neutral idle speed Ni. Whenever the ratio indicates that negative (braking) torque is being transmitted across the torque converter 24, the decision block 558 is executed to determine whether the current turbine speed N_t is greater than the stored neutral idle speed Ni. If the stored neutral idle speed is correct, the decision block 558 will be answered in the affirmative. However, if the stored neutral idle speed is too high, the decision block 558 may be answered in the negative. In such case, the instruction block 560 is executed to average the current turbine speed N_t into the stored neutral idle speed, to bring the stored value into line with the actual neutral idle speed. The condition of negative (braking) torque is determined by the decision blocks 544 and 562, which detect when the ratio N_t/N_e is between the terms COASTHR and DRIVETHR, and the decision block 548, which further detects whether the ratio is greater than the reference term HI. If the decision block 562 is answered in the negative, negative (braking), torque is not indicated, and the instruction block 564 is executed to clear the "COASTBIT" flag.
In the above manner, errors of neutral idle speed over-estimation (stored neutral idle speed too high) can be at least partially corrected early in the coast operation prior to the capture of the actual neutral idle speed. Errors of under-estimation (stored neutral idle speed too low) are corrected solely by the neutral idle speed capture as described above in relation to the instruction block 554.
The shift timing routine 498 is initiated by comparing the predicted turbine speed for the downshifted speed ratio PTSn with the stored neutral idle speed Ni, as indicated by the instruction block 566. When PTSn is less than or equal to Ni and the "ACTIVE CDS" flag is not set, as determined at the decision block 568, the downshift is initiated by executing the instruction blocks 570 to 572 to set the "ACTIVE CDS" flag, to set the desired speed ratio R_des equal to the coast downshift ratio Reds, and to decrement Reds.
If it is determined at the decision block 568 that the "ACTIVE CDS" flag is set, another coast downshift is already in progress, and the instruction block 574 is executed to force such shift to its logical conclusion by setting the desired speed ratio R_des equal to the actual ratio R_act, clearing the "SHIFT IN PROGRESS" flag, resetting the "FILL START flag, and decrementing the coast downshift ratio Reds. This situation occurs under relatively high rates of deceleration, as illustrated in Figure 7, where the downshift to an intermediate forward speed ratio, for example second ratio, is skipped.
If it is determined at the decision block 566 that the predicted turbine speed PTS_n is greater than the stored neutral idle speed Nj, the decision block 576 is executed to determine whether the "ACTIVE CDS" flag is set. If not, the remainder of the routine is skipped, as indicated by the flow diagram line 578. If so, the blocks 537 and 538 are executed, to compare the current speed ratio Pact with the desired speed ratio R_des, and to clear the "ACTIVE CDS" flag if it is determined that the desired speed ratio R_des has been achieved.
The blocks in Figure 9 generally designated by the reference numeral 584 set forth an approach which may be used in place of the approach set forth at instruction block 566. Essentially, the moment for releasing the active clutching device may be identified either by computing a predicted turbine speed PTS_n and comparing it with the stored neutral idle speed Ni, as described above, or by computing a neutral idle speed offset and comparing it with the turbine speed in the downshifted speed ratio. As is set forth at the instruction block 585, the neutral idle speed offset is computed as a function of the stored neutral idle speed Ni, the vehicle acceleration a, the predetermined time Tp (0.25 sec), and the downshifted speed ratio. The turbine speed in the downshifted speed ratio N_tds is computed as a function of the current vehicle speed N_v and the ratio N_t/N_o defined by the downshifted speed ratio, as indicated at instruction block 586. When the turbine speed in the downshifted speed ratio equals the neutral idle speed offset, as determined at the decision block 587, the decision block 568 is executed to determine whether the "ACTIVE CDS" flag is set as described above.
As is indicated above, the flow diagrams of Figures 10 and 11 set forth the clutch and PRV pressure determination algorithm generally referred to at the main loop instruction block 478 of Figure 8. On entry into such algorithm, the blocks designated generally by the reference numeral 588 are executed, to set up initial conditions if a shift is in order. If a shift is in order, the blocks designated generally by the reference numeral 590 are executed, to develop pressure commands for the clutching devices involved in the shift. Thereafter, the instruction blocks 592 and 594 are executed, to determine pressure commands for the non-shifting clutches and the pressure regulator valve PRV, so completing the routine. As is indicated at the instruction block 594, the pressure command for the regulator valve PRV is set to be equal to the highest of the pressure commands for the various clutching devices.
The blocks designated by the reference numeral 588 include the blocks 596 to 604. The decision block 596 determines whether a shift is in progress, as indicated by the "SHIFT IN PROGRESS" flag; the decision block 598 determines whether the actual speed ratio Pact is equal to the desired speed ratio R_des determined at the instruction block 476 of Figure 8; and the instruction block 600 sets up the initial conditions for a ratio shift. The decision block 602 and the instruction block 604 set up an initial condition for a coast downshift. The blocks 600 to 604 are executed only when the decision blocks 596 and 598 are both answered in the negative. In such case, the instruction block 600 serves to set the old ratio variable, Rold, equal to Pact to set the "SHIFT IN PROGRESS" flag, clear the shift timers, and to calculate the fill time t/ill for the on-coming clutching device. Then the decision block 602 determines whether the "ACTIVE CDS" flag is set. If so, the instruction block 604 is executed, to set the CDS FILL START TIMER to the difference (Tp - t_fill). If the "ACTIVE CDS" flag is not set, the execution of the instruction block 604 is skipped, as indicated by the flow diagram line 606. If a shift is in progress, the execution of the blocks 598 to 604 is skipped, as indicated by the flow diagram line 608. If no shift is in progress, and the decision block 598 is answered in the affirmative, the execution of the blocks 600 to 604 and the blocks designated by the reference numeral 590 is skipped, as indicated by the flow diagram line 610.
The blocks designated by the reference numeral 590 include the decision block 612 for determining whether the shift is an upshift or a downshift; the instruction block 614 for developing pressure commands for the on-coming and off-going clutching devices if the shift is an upshift; and the instruction block 616 for developing the pressure commands for the on-coming and off-going clutching devices if the shift is a downshift. Since the present invention is concerned with downshifting, the steps involved in the downshift logic and control function identified by the instruction block 616 are set forth in greater detail in the flow diagram of Figure 11.
On entry into the flow diagram of Figure 11, the decision block 620 is executed to determine whether the fill phase of the shift has been completed, as is indicated by the "FILL COMP" flag. If not, the flow diagram branch generally designated by the reference numeral 622 is executed; if so, the flow diagram branch generally designated by the reference numeral 624 is executed.
The flow diagram branch 622 includes a fill initialising routine comprising the blocks 626 to 632, and a fill completion routine comprising the blocks 638 and 640. At the beginning of each shift, the "FILL COMP" flag is not set, and the decision block 626 of the fill initialising routine is executed, to determine whether the fill phase has started, as indicated by the "FILL START" flag. Initially the "FILL START" flag is not set, and the decision block 628 is executed to determine whether the "ACTIVE CDS" flag is set. If not, the instruction block 630 is executed, to set the energisation duty cycle of the on-coming clutching device, DC(ONC), equal to 100 %, to set the "FILL START flag, and to start the FILL TIMER. If the "ACTIVE CDS" flag is set, the decision block 632 is executed to determine whether the count in the CDS FILL START TIMER is zero. If not, the predetermined time Tp (0.25 sec) has not expired, and the remainder of the fill routine is skipped, as indicated by the flow diagram line 634. If the count is zero, the instruction block 630 is executed, to initiate the fill phase of the downshift as described above. Thereafter, the decision block 626 is answered in the affirmative, and execution of the blocks 628 to 632 is skipped, as indicated by the flow diagram line 636.
The decision block 638 of the fill completion routine determines whether the count in FILL TIMER is greater than or equal to the fill time tfix determined at the instruction block 600 of Figure 10. If so, the instruction block 640 is executed to set the "FILL COMP" flag. If the decision block 638 is answered in the negative, the fill phase is incomplete, and execution of the instruction block 640 is skipped, as indicated by the flow diagram line 642.
The flow diagram branch 624 includes a shift initialising routine comprising the blocks 644 to 650, and a shift completion routine comprising the blocks 652 to 662. The decision block 644 of the initialising routine determines whether the "FILL COMP" flag has just been set, as indicated by the status of the "FIRST FILL" flag. If so, the instruction blocks 646 and 648 are executed, to set up the torque and inertia phases of the shift. The instruction block 646 determines the pressure parameters Pi, P_f and tt for the on-coming (ONC) and off-going (OFG) clutching devices as a function of an engine torque-related variable Tv, the parameters Pi, P_f, and t_f being defined in Graphs C and D of Figure 4. Such determination is set forth in detail in EP-A-0 231 523.
The instruction block 648 starts an inertia phase timer IP TIMER, and resets the "FIRST FILL" flag. Thereafter, the decision block 644 is answered in the negative, and the instruction block 650 is executed to calculate the percentage of ratio shift completion, %RATCOMP.
In the inertia phase completion routine, the decision blocks 652 and 654 are executed, to determine whether the count in IP TIMER is at a maximum value, MAX, or whether the term %RATCOMP is substantially equal to 100 %. If either of the decision blocks 652 or 654 is answered in the affirmative, the shift is complete, and the instruction block 656 is executed, to reset the "SHIFT IN PROGRESS" flag, to clear the "ACTIVE CDS" flag, to set the on-coming duty cycle DC(ONC) equal to 100%, and to set the off-going duty cycle DC(OFG) equal to 0 %. If both of the decision blocks 652 and 654 are answered in the negative, the instruction block 658 is executed, to determine the on-coming and off-going pressure commands, P(ONC) and P(OFG), as a function of the Pi, Pt, tt and IP TIMER values. This function is also set forth in detail in the said EP-A-0 231 523. Thereafter, the blocks 660 to 662 are executed, to set the pressure command for the active (off-going) clutching device to zero if the "ACTIVE CDS" flag is set.
As is set forth above, the coast downshift control of this invention effects minimal driveline disruption downshifts and places the transmission in a suitable speed ratio throughout the course of the coast operation, so that further downshift need not necessarily occur if and when the operator terminates the coast operation.

Claims

1. A method of controlling apply and release of transmission friction elements to effect shifting of a transmission from an upper speed ratio to a lower speed ratio as vehicle speed is reduced in the course of a coast mode of operation (closed throttle deceleration), in a motor vehicle having an engine (12) drivingly connected to a vehicle wheel by way of a fluid coupling (24) and a multiple-speed ratio transmission, where the fluid coupling (24) includes an impeller (36) connected to the engine (12) and a turbine (40) connected to the transmission (14), the transmission (14) being shiftable from an upper speed ratio to a lower speed ratio by release of a friction element (28 to 32) associated with the upper speed ratio and apply of a friction element (28 to 32) associated with the lower speed ratio, and the motor vehicle being operable in a coast mode of operation wherein the vehicle wheel and transmission (14) drive the turbine (40) faster than the engine (12) drives the impeller (36) until the engine speed falls below its no-load idle speed, after which the engine (12) drives the impeller (36) faster than the vehicle wheel and transmission (14) drive the turbine (40), characterised in that the method comprises the steps of: maintaining the apply of the friction element (28 to 32) associated with the upper speed ratio at least until the engine speed is driven below the no-load idle speed; and thereafter releasing the transmission friction element (28 to 32) associated with the upper speed ratio and subsequently applying the transmission friction element (28 to 32) associated with the lower speed ratio such that:

1. in the neutral interval between such release and such subsequent apply the engine speed substantially returns to the no-load idle speed, and

2. the engine speed immediately after the apply of the transmission friction element (28 to 32) associated with the lower speed ratio is substantially equal to said no-load idle speed, whereby torque fluctuations at the vehicle wheel are minimised.

2. A method according to claim 1, characterised in that the method includes the step of determining as a function of the current vehicle speed the speed at which the vehicle wheel and transmission (14) would drive the turbine (40) assuming engagement of the lower speed ratio, and predicting as a function of the current vehicle deceleration the value of such speed a predetermined time interval in the future, such predetermined time interval being determined in relation to the characteristic time required for the engine speed to return to its no-load idle speed from a lower value when the engine load is removed, and that the release of the friction element (28 to 32) associated with the upper speed ratio is effected so as to remove the engine load when the predicted turbine speed coincides with the no-load idle speed of the engine (12), thereby initiating a neutral interval in which the engine speed increases substantially to its no-load idle speed, and the subsequent apply of the friction element (28 to 32) associated with the lower speed ratio is effected a predetermined time interval thereafter such that engagement of the friction element (28 to 32) substantially coincides with the substantial return of the engine (12) to its no-load idle speed.

3. A method according to claim 1, characterised in that the method includes the step of computing as a function of the current vehicle speed the . speed at which the vehicle wheel and transmission (14) would drive the turbine (40) assuming engagement of the lower speed ratio, and predicting as a function of the current vehicle deceleration the value of such computed speed a predetermined time prior to its reaching the neutral idle speed of the engine (12), such predetermined time bearing a relation to the characteristic time required for the engine speed to return to its no-load idle value from a lower value when the engine load is removed, and that the release of the friction element (28 to 32) associated with the upper speed ratio is effected when the computed turbine speed reaches the predicted turbine speed, to thereby initiate the neutral interval during which the engine speed increases towards its no-load idle speed, and the apply of the friction element (28 to 32) associated with the lower speed ratio is effected a predetermined time after the release of the friction element (28 to 32) associated with the upper speed ratio such that such apply substantially coincides with the return of the engine (12) to its no-load idle speed.

4. A method according to any one of claims 1 to 3, characterised in that the method includes the step of determining the no-load idle speed of the engine (12) at the onset of the coast mode of operation by monitoring a relation respecting the speeds of rotation of the impeller (36) and turbine (40), and identifying the engine speed at which the turbine (40) and impeller (36) are driven at substantially the same speed.

5. A method according to any one of claims 1 to 3, characterised in that the method includes the steps of:

estimating the neutral idle speed of the engine (12) at the initiation of engine operation and storing an indication representative thereof for use in timing the release of the friction element (28 to 32) associated with the upper speed ratio; monitoring a relation respecting the speeds of rotation of the impeller (36) and turbine (40) at the onset of the coast mode of operation, and identifying the actual no-load idle speed at which the speeds of rotation of the turbine (40) and impeller (36) substantially coincide; and

adjusting the stored indication of neutral idle speed by an amount determined in relation to the identified actual neutral idle speed, thereby to update the stored indication in accordance with current engine operating conditions.

6. A method according to claim 1, characterised in that the transmission (14) is adapted to provide a low gear providing one forward speed ratio and at least two other gears providing successively higher forward speed ratios, the transmission (14) is shiftable from a current speed ratio to a desired speed ratio by release of a friction element (28 to 32) associated with such current speed ratio and apply of a friction element (28 to 32) associated with such desired speed ratio, the method is effective to control the apply and release of the transmission friction elements (28 to 32) to effect successive shifting of the transmission (14) from one of the said other gears to the low gear as the vehicle speed is reduced in the course of the coast mode of operation, and the method includes the steps of:

maintaining the apply of the friction element (28 to 32) associated with the gear engaged at the onset of the coast mode of operation at least until the engine speed is driven below the no-load idle speed; and thereafter releasing the friction element (28 to 32) associated with such gear and subsequently applying the friction element (28 to 32) associated with a gear which provides a lower speed ratio than the engaged gear until the low gear is engaged, each such release and apply being carried out such that:

1. in the neutral interval between the release and subsequent apply of respective friction devices (28 to 32) the engine speed substantially returns to its no-load idle speed, and

2. the engine speed immediately after the applying of each such friction element (28 to 32) is substantially equal to said no-load idle speed.

7. A method according to claim 6, characterised in that the method includes the step of:

inhibiting the apply of the friction element (28 to 32) associated with any of the said other gears which provide a lower speed ratio than the engaged gear in the course of the coast mode of operation if the release of such friction element (28 to 32) is required prior to its apply, thereby to extend the neutral interval and skip the engagement of one or more of the said other gears in the presence of relatively high vehicle deceleration.

8. A method according to claim 1, characterised in that the transmission (14) is adapted to provide a low forward speed ratio and at least two successively higher forward speed ratios, the transmission (14) is shiftable from a current speed ratio to a desired speed ratio by release of a friction element (28 to 32) associated with such current speed ratio, and apply of a friction element (28 to 32) associated with such desired speed ratio, the method is effective to effect successive shifting of the transmission (14) from the current forward speed ratio to the low forward speed ratio as the vehicle speed is reduced in the course of the coast mode of operation, and the method includes the steps of:

beginning at the onset of the coast mode of operation, computing the speed at which the vehicle wheel and transmission (14) would drive the turbine (40) assuming engagement of a successively lower transmission speed ratio, and predicting as a function of the current vehicle deceleration the value of such speed a predetermined time interval in the future, such predetermined time interval bearing a relation to the characteristic time required for the engine speed to return to its no-load idle value from a lower value when the engine load is removed;

releasing the friction element (28 to 32) associated with the currently engaged speed ratio to remove the engine load when the predicted turbine speed for the successively lower speed ratio coincides with the no-load idle speed of the engine, thereby initiating a neutral interval during which the engine speed increases substantially to its no-load idle speed; and

applying the friction element (28 to 32) associated with the successively lower transmission speed ratio a predetermined time after the release of the friction device (28 to 32) associated with the currently engaged speed ratio unless the predicted turbine speed for the next successively lower transmission speed ratio coincides with the no-load idle speed of the engine first, thereby to extend the neutral interval and skip the engagement of the successively lower transmission speed ratio when the vehicle deceleration is relatively high.

9. A method according to claim 1, characterised in that the transmission (14) is adapted to provide a low forward speed ratio and at least two successively higher forward speed ratios, the transmission (14) is shiftable from a current speed ratio to a desired speed ratio by release of a friction element (28 to 32) associated with such current speed ratio and apply of a friction element (28 to 32) associated with such desired speed ratio, the method is effective to effect successive shifting of the transmission (14) from the current forward speed ratio to the low forward speed ratio as the vehicle speed is reduced in the course of the coast mode of operation, and the method includes the steps of:

beginning at the onset of the coast mode of operation, computing the speed at which the vehicle wheel and transmission (14) would drive the turbine (40) assuming engagement of a successively lower transmission speed ratio, and predicting as a function of the current vehicle deceleration the value of such computed speed a predetermined time prior to its reaching the no-load idle speed of the engine (12), such predetermined time bearing a relation to the characteristic time required for the engine speed to return to its no-load idle value from a lower value when the engine load is removed;

releasing the friction element (28 to 32) associated with the currently engaged speed ratio to remove the engine load when the computed turbine speed for the successively lower speed ratio reaches the predicted turbine speed for such successively lower speed ratio, to thereby initiate a neutral interval during which the engine speed increases substantially to its no-load idle speed; and

applying the friction element (28 to 32) associated with the successively lower transmission speed ratio a predetermined time after the release of the friction device (28 to 32) associated with the currently engaged speed ratio unless the computed and predicted turbine speeds for the next successively lower transmission speed ratio coincide, thereby to extend the neutral interval and skip the engagement of the successively lower transmission speed ratio when the vehicle deceleration is relatively high.